TCAS Questions

[davethelimey]

I write in reference to this BBC article (http://news.bbc.co.uk/1/hi/uk/3411941.stm) , which is the story of a near-miss 15 months ago between a Delta 767 and a Virgin 747. Apparantly at one point the vertical seperation was 100 feet and laterally, 1.5 nm. I have two questions:

1. The pilot of the Virgin plane pulled up in accordance with the TCAS system - does the alarm go off when planes are definitely about to be in exactly the same place at the same time, or is there a margin of error, say 100 feet vertically? Basically, is the TCAS set to go off only when there is a 100% chance of collision?

2. Say the closest the planes ever got vertically was 100 feet, and the Delta passes right under the 747. Given the size of the planes, what (if any) would the effect be - jetwash, turbulance etc?

Many thanks to anyone with an answer.

Dave.

[ratarsedagain]

Dave,
In response to your questions,

1. TCAS identifies a 3-dimensional piece of airspace around your aircraft where a possibility of traffic conflict exists. The size of this bit of airspace depends on the rate of closure of any conflicting aircraft. A Resolution Advisory (RA), (the highest level alert) indicates that TCAS has calculated that another aircraft will enter the conlict airspace within approx. 20-30 seconds.

2. I've never got that close to another aircraft in flight, and hopefully never will, so can't give an answer to that, however a change of underwear would probably be required on both flight decks!

Cheers!

[davethelimey]

Good answer!

Many thanks.

[europilot]

Today I was flying at around 6000ft in the outback of Australia (definitely no radar coverage) and as usual my IDENT sign on the transponder wasn't blinking.
I saw a jet coming towards me on converging heading somewhere up in flight levels and the IDENT started flashing...

What's the vertical/horizontal range of the TCAS system in heavy jets, I thought it was only 2000-3000 feet above and below???

Any good answers are appreciated!!!

EP

[Flex33]

According to the FCOM an aircraft is considered a threat within 30nm +- 2700ft. However, if we select 'ABOVE' the system scans +9900ft & -2700ft, just transpose those numbers for 'BELOW'. These figures apply to climb, cruise & descent.
There are other modes that the system adopts depending on the stage of flight.

[Phoenix_X]

The interrogation signal doesn't get aimed either. Depending on many factors, the interrogation signal from the TCAS equipped aircraft could reach aircraft lower than the 'scan' range, which would have your light blinking. And ofcourse, your xponder responding to it, which may or may not be received by the TCAS aicraft (again depending on many factors). Only if it is received, THEN will it be filtered if it is outside the scan range :)

[hawk37]

Euro: the jet with the tcas will be constantly interrogating. When your transponder picks it up, it will reply. I think the jet’s transponder will be much weaker than atc’s, so that may be why your transponder doesn’t flash at long distances.
My experience is with an Allied Signal tcas 67, and this response is based on that unit in our aircraft only.
As people have said, whether you are shown on the tcas display depends on whether the jet can pick up your reply, and whether the altitude difference meets the preset requirements to be displayed.
If you are not squawking altitude, then you will show up on the jet’s tcas display anytime you are within range, with a strong enough signal for the jet’s transponder antennas to detect your signal. The jet will also get a proximity alert (filled diamond), followed by a traffic advisory (yellow circle, and "traffic traffic" audio). Since there is no altitude, no climb or descent commands can be issued.
Operationally, when we get this, we see that there is no altitude reported, so understand that the tcas does not know your altitude. Normally, these are low vfr targets, which we can sometimes see.

AND NOW, the big question, Why do high flying tcas equipped aircraft not see a myriad of low level vfr type aircraft that are not squawking mode C?

I believe the answer may be two fold.
1. The low vfr traffic may not receive the high flyers interogation, since antenna on the low vfr aircraft are oriented for a ground atc interrogation. EURO, DO YOU REPLY CONTINUOUSLY to these jets, until they are well past? OR does your reply cease before?
2. TCAS antennas are not particularly suited to receiving responses from aircraft WELL below them. In fact, in some cases we’ve had a "no bearing" traffic advisory a few times, where the tcas is unable to compute the bearing. This is even with the gear up, so upper and lower directional antennas should be in operation.


Of course, I could be wrong.
Hawk

[Cougar]

Our TCAS II System (speaking from my personal aircraft only) has a lateral range of: 30Nms forwards, 15Nms backwards and 12 Nms to the side. With ETCAS on this boosts out to 60Nms forwards. Vertically in above or below we are looking at up to 7700ft, but in ETCAS this goes out to 12000ft.

As for the question of how sh*tscared would the virgin and delta pilots be, when you consider that the box is taking its height to be where it is in space, and not the highest point on the aircraft, and the tail height of a 767 is 55ft, and the lowest point of the fuselage of a 747 is roughly 25ft below the avionics bay, when the TCAS shows 100ft the aircraft could possibly be only 20 ft apart vertically. If you disregard the fact that they were 1.5Nms apart laterally, its bloody close!!

[zerozero]

Europilot mentioned his lack of radar contact in the Australian Outback.

I used to fly in southwestern Alaska and was under the impression no one was painting me on any radar screen--and then I learned the entire coast of Alaska is covered with military radar--they just don't share with ATC.

All that time I thought I was sneaking around the coastline all by myself but some punk in Anchorage was watching me in a dark warm room having a cup of coffee and a cigarette.

:hmm:

[ICT_SLB]

As stated by previous posters, the exact distance you get a Resolution Advisory (the command to climb or descend to avoid the intruder) is dependent on the software load. The older 6.04 gave you an alert whenever the flight path came within about 1 NM (this varies a lot with closure rates & relative altitudes) but the later Change 7 is designed NOT to give a false alert when aircraft are in RVSM airspace (as close as 1000ft apart in altitude). Under test conditions (trying to get alerts on the TCAS display on a HUD), it was found you had to be pretty much on a collision course before an RA occurred if Change 7 was loaded.

TCAS uses "whisper/shout" to distinguish between potential intruders. Targets which transmit returns when a high-power interrogation is sent out are ignored for the purposes of conflict management if they do not reply to the low power "whisper" as they are assumed to be out of conflict range. Only those intruders that are tracked and enter the warning zone - the "Tau" - will cause a "TRAFFIC" alert and subsequently give rise to an RA.

[CJ Driver]

Slightly off topic, but I notice that the original "scary story" actually had the two aircraft 1.8 nm apart at their closest point.

I know I probably need a new prescription for my spectacles, but without a contrail, and with some ground clutter in the background, I suspect I would never have even seen someone nearly two miles away, let alone worried about hitting them. :ooh:

Since TCAS issued an RA for this one, I would deduce that (1) the advised climb was probably quite modest and (2) that TCAS is fairly conservative.